1. Field of the Invention
The present invention relates to a chemical vapor deposition apparatus and, more specifically, to a chemical vapor deposition apparatus having a purifier for purifying a gas such as AsH.sub.3 or PH.sub.3 gas used in chemical vapor deposition.
2. Description of the Related Art
FIG. 2 schematically shows a conventional metal organic chemical vapor deposition (MOCVD) apparatus, such as that shown, for example, in "Appl. Phys. Lett., 42(1), pages 83 to 85 (1983)" or "Appl. Phys. Lett., 41(1), pages 88 to 90 (1982)". As shown in FIG. 2, a H.sub.2 gas cylinder 1 is connected by a pipe 2a to an organometal cylinder device 3 connected with a reactor 4 so that organometal vapor from the organometal cylinder device 3 is conveyed by H.sub.2 gas and introduced into the reactor 4. H.sub.2 gas is introduced from the H.sub.2 gas cylinder 1 through another pipe 2b into the reactor 4 . An AsH.sub.3 or PH.sub.3 gas cylinder 5, serving as a raw-material gas supplying means, is connected by a pipe 2c to a gas purifier 6 so that AsH.sub.3 or PH.sub.3 gas from the cylinder 5 is introduced to the purifier 6, and purified thereby. The purified raw-material gas is introduced through a pipe 2d into the reactor 4. After a reacting process, the gases, such as unreacted gases, remaining in the reactor 4 is exhausted to the outside of the apparatus by an exhausting means 7.
With the conventional chemical vapor deposition apparatus having the above-described construction, the organometal vapor conveyed by H.sub.2 gas into the reactor 4 is mixed with AsH.sub.3 or PH.sub.3 gas and heated, whereby these substances decompose and react with each other so that the resultant product precipitates to form a crystal. When, for instance, an AlGaAs-based crystal is to be grown, trimethyl aluminum (TMA) and trimethyl galium (TMG) are reacted with AsH.sub.3. When an InP-based crystal is to be grown, trimethyl indium (TMI) is reacted with PH.sub.3.
In order to increase the purity of the crystal which is to be obtained, AsH.sub.3 or PH.sub.3 gas from the raw-material supplying means is introduced to the gas purifier 6. After substances such as H.sub.2 O and O.sub.2 are removed from the AsH.sub.3 or PH.sub.3 gas, the raw-material gas is introduced into the reactor 4. The gas purifier 6 has a vessel containing a suitable low-melting-point metal, such as AlGaIn, which is in its liquid state at normal temperature. The raw-material gas (AsH.sub.3 or PH.sub.3) is bubbled into the low-melting-point metal (AlGaIn).
Such a conventional chemical vapor deposition apparatus entails the following problem: vapor of Al, Ga, In, or compounds thereof may be introduced into the reactor 4 from the low-melting-point metal AlGaIn used in the gas purifier 6. If vapor of Al or the like are introduced into the reactor 4 during the growth of an AlGaAs- or InP-based crystal, the crystal structure may be adversely influenced, or the introduced vapor may cool and solidify in the pipe 2d, thereby clogging the pipe.